The present invention relates to a pump suction pipe, and particularly to a pump suction pipe having a bent portion.
In a pumping station such as a drainage pumping station, water sucked from a water channel through a suction pipe is pressurized by a pump body in many cases. In this case, the suction pipe has not only a straight pipe portion but also a bent pipe portion. An example of a pump suction pipe having such a bent pipe portion is described in Japanese Unexamined Utility Model Application Publication No. H1-76597/1989 and Japanese Unexamined Utility Model Application Publication No. S58-33887/1983.
Japanese Unexamined Utility Model Application Publication No. H1-76597 describes that in order to guide regulated water to a pump while suppressing the drift of a flow flowing into a suction pipe, a suction port is opened towards the inflow direction in a vertical pump, and a regulation bent pipe bent towards the upper direction is provided at a tip end of a suction portion. In this case, the regulation bent pipe is configured like a bent pipe, and is formed in a curve line (curve surface) in accordance with the curvature of the bent pipe slightly on the inner side relative to a central axis of the bent pipe, namely, on the side where the radius of curvature is small.
Japanese Unexamined Utility Model Application Publication No. S58-33887 describes that in a suction pipe channel of a pump in which a center line of a suction port of the pump is in a horizontal direction or nearly in a horizontal direction and which suction inlet is submerged in the fluid through a suction bend pipe formed vertically or with an almost vertical angle, a divider is provided at the bent pipe to improve the flow of the fluid from the bent pipe to the pump. Further, the divider is bent on the side near the pump. The cross-sectional area of the outer flow channel is the largest on the outlet side, namely, on the side of the pump, and the cross-sectional area of the inner flow channel is the smallest on the outlet side, namely, on the side of the pump among the flow channels divided by the divider.
Further, a bend with guide vanes is disclosed as a bent pipe used for such a suction pipe in “JSME Mechanical Engineers' Handbook: Fundamentals α4: Fluid Engineering”, First Edition, The Japan Society of Mechanical Engineers, January, 2006, α4-PP. 73, 77, 78. In order to decrease a loss in the bend while suppressing a secondary flow or flow separation generated in the bend, the document describes that guide vanes obtained by bending thin plates into an arc shape with a central angle of 90 degrees are concentrically inserted into the bend, and are attached to the position where partial flow channels divided by the guide vanes have the same radius ratio.
Incidentally, as described in “10 Articles of Fluid dynamics” written and edited by Masakazu, HARADA, First Edition, Yokendo co. Ltd., February, 1989, p. 42, the velocity of a flow flowing into a bent pipe is slow near a wall surface of a flow channel due to friction between the wall surface of the flow channel and a liquid (water), and is fast in the middle of the flow channel. When the flow passes through a bent portion, the centrifugal force is applied to the liquid. The centrifugal force increases in proportion to the square of the velocity in the arc-like direction (arc circumferential direction) along the bent portion, and is applied in the direction (arc radial direction) from the inner circumferential side to the outer circumferential side of the bent portion.
As a result, the mainstream of the liquid in the middle of the flow channel of the bent portion flows from the center of the flow channel of the bent portion towards the outer circumferential side by the action of the centrifugal force as described in “10 Articles of Fluid dynamics”. Further, a pressure gradient in the radial direction of the arc occurs in the liquid bent like an arc by passing through the bent portion due to the centrifugal force applied in the radial direction of the arc. In this case, the pressure is high on the outer circumferential side, and low on the inner circumferential side.
On the other hand, in the flow passing through the bent portion, a boundary layer is formed near the wall surface where the velocity in the circumferential direction of the arc is slower than that of the mainstream in the middle of the flow channel. The flow in the boundary layer cannot match, in the circumferential direction of the arc, the mainstream in the middle of the flow channel that becomes a flow flowing from the center of the flow channel to the outer side due to the action of the centrifugal force, and forms a flow flowing from the outer circumferential side where the pressure is high to the inner side where the pressure is low along the wall surface. In addition, in the cross-section orthogonal to the central axis of the bent portion, a secondary flow flowing from the center of the flow channel to the outer side is formed in the middle of the flow channel, and the secondary flow flowing from the outer side to the inner side along the wall surface is formed near the wall surface.
The secondary flow is similarly generated in a pump suction pipe having a bent portion. Further, if a liquid flows into an impeller suction port after passing through the bent portion while the secondary flow remains in the pump suction pipe, an area of a discrepancy between the inflow angle of a liquid at the impeller suction port and the blade angle of the impeller is generated at the impeller suction port in the circumferential direction relative to the rotational axis of the impeller even at the design point in some cases.
In the area where a discrepancy between the inflow angle of a liquid at the impeller suction port and the blade angle of the impeller is large, the liquid near a leading edge of the impeller blade does not flow in along the blade, but flows around a tip end of the blade. In the area where the liquid flows around a tip end of the blade, the relative velocity of the liquid to the impeller is locally increased, and the pressure is decreased. As a result, under the pump operation conditions where the pressure of the inlet of the impeller is low, cavitation is likely to be locally generated in the area where the liquid near a leading edge of the impeller blade flows around a tip end of the blade.
Further, due to an influence of the secondary flow generated at the bent portion to reach the impeller suction port, the areas where a difference between the inflow angle of a liquid at the impeller suction port and the blade angle of the impeller is large are disproportionately generated in the circumferential direction relative to the rotational axis of the impeller, and areas of the cavitation that is locally generated as described above are disproportionately generated as similar to the above. As a result of generation of the cavitation in the disproportionate areas, fluctuating force is loaded on the impeller due to a density difference between a gas area of cavitation with low density and a normal fluid area, and large oscillation and noise are likely to be generated in the pump.
In the conventional suction pipe of the pump described in Japanese Unexamined Utility Model Application Publication No. H1-76597 or Japanese Unexamined Utility Model Application Publication No. S58-33887, or the bend described in “JSME Mechanical Engineers' Handbook”, the secondary flow flowing from the inner side to the outer side is suppressed in the middle of the flow channel of the bent portion by employing the bent pipe with a divider and the regulation bent pipe. However, if the divider is additionally provided, designing, processing, and construction become complicated, leading to an increase in cost. Further, in an operation at the non-design point (an area of a low flow rate), it is generally well known that a recirculation flowing from the impeller to the suction side is generated on the shroud side of the impeller. If the recirculation area becomes large, the recirculation area reaches as far as a divider or a regulation plate. In this case, the recirculation hits the divider or the regulation plate to generate oscillation or noise that possibly damages the divider or the regulation plate.